O.k. I've got some questions about what is happening in a tube amplifier when the speakers are run off the 4 ohm taps instead of 8 ohm taps.

On a SS amp, this is not an option, though depending on the amplifier there is sometimes concern about running a pair of 4 ohm speakers in parallel. I've got a couple of pro sound amps used by my youth group that can run into 2 ohm loads all night and not sweat it.

I think I can remember Craig suggesting to discover whether I preferred the sound of my Scalas running off the 4 or 8 ohm taps of my first 299. Never did check that out, seemed too much like work.

From time to time I will set up a Heresy center with my Khorns and run the output through a Dynaco QD2 to derive a center channel and run all three speakers off two monoblock amps. The sound is impressive.

Someone shared with me a week ago that the rigged as such, the QD2 presents a 6 ohm load to the amps. So I thought, might be a good time to change to the 4 ohm taps to see what it sounds like. I switched and am running the Wright 2A3 monoblocks off the 4 ohm taps through the QD2 to the Khorns and Heresy center. Wow.

To my ears, the whole sound, especially the mid bass and bass sounds easier, fuller, richer, effortless. I didn't really know what to expect.

One thing I'm wondering is... what is going on in the amplifier running of 4 ohm instead of 8 ohm taps?

How does a SS amp manage w/o different taps?

What if one runs 8 ohm speakers off 4 ohm taps all the time?

Is the difference in sound completely subjective? Could it be measured with instruments?

My daughter (in engineering school) just gave me her textbook from an intro to electronics she had this semester. So I'm in kindergarten.

Appreciate any help here. Talk real slow please.

The short story is that the output transformer is there to keep the load even on the output tubes with the different speakers. If you hooked up a 4 ohm speaker on 8 ohm taps, the output tubes will see twice the power draw (current) than normal. When you hook up to the 4 ohm taps, the load is back to normal for the output tubes. Tubes are voltage devices, not current. If I have this wrong, I am sure that I will be corrected! The output on a SS amp, transistors, are current devices not voltage. If they are big enough they can suck up the extra load that a speaker mismatch puts on them. Again I am sure if I am wrong, there will be voices to that affect. At least this is the way I understand the situation.

The transformer is an impedance lever. The impedance you put on the output is "reflected back" as the "load" to the output tubes based on a FIXED MULTIPLIER.

That "multiplier" is the square of the turns ratio between the primary side and each secondary tap. Assuming the primary is "5,000" ohms, a typical figure for a pair of output tubes, then the 8 ohm tap has a 25:1 turns ratio giving a "multiplier" of 625.

This means, NO MATTER WHAT impedance you put on the 8 ohm tap, it will reflect BACK 625 times that impedance onto the output tubes. So, if you put 6 ohms on that 8 ohm tap, it will reflect back 6 X 625 = 3,750 ohms to the output tubes. If you put 9 ohms on that tap it will reflect back 9 X 625 = 5,625.

The 4 ohm tap has a turns ratio of 35 and a multiplier of 1225. So, putting 6 ohms on there will load the output tube with 7350 ohms.

So, what's the point? Well, the transformer is designed to provide the ideal LOAD to the output tubes - in this example, 5000 ohms, which will happen only when 4 ohms are on the 4 ohm tap or 8 ohms are on the 8 ohm tap or 16 ohms are on the 16 ohm tap. When the output tube "LOAD" is too low below 5000 ohms you load down the tubes, increase the distorton. When the output load is too high above 5000 ohms you severely limit the power output.

The point to remember is that the transformer has no impedance of it's own, it only multiplies impedance of the objects connected to it, and that multiplication ratio is fixed, while what you attach is variable.
mdeneen

Bravo Mark - well done and clear...

Dee, your Klipschorns and LaScalas have a nominal impedance of 6 ohms. There some large swings in impedance, going from a little over 5 ohms all the way up to 30 ohms or more. Though sensitive speakers, our big horns actually present a somewhat complex and difficult load to a tube amp. I would use the 4 ohm taps, and if you want to do something interesting -- attach a 15 ohm resistor in parallel with the amps outputs.

I thought the impedance was usually given as 8 ohms. No matter, whatever setting you choose is a compromise because the ohm rating listed is just in name only. In the "Audio" magazine review of the Klipschorn in 1986, the Klipschorn tested ranged from 4.5 ohms to 42.3 ohms, depending on frequency. The measurement constantly changes depending on the music being played.

One big benefit of the ALK balancing networks is that this range is tightened a lot.

Bass performance should indeed be better using the 4 ohm connections on your amp.

Although the speaker impedance swings dramatically, this is not as scary as it sounds, because as the driver reaches say 40 ohms, it is also reaching it's resonance - which means it takes very LITTLE power to produce the constant sound pressure.

If the amp "didn't care" about impedance, the "power out" would be uniform at all frequencies. And, for the most part, this is true. Where it is not true, is when you try to extract MAXIMUM POWER from the amp, and this brings it back to transfomers again.

So, the transfomer is always has equal power on each side of the winding. 25 watts on primary will be 25 watts on secondary. Since power is the product of voltage and current, you can surmise that the transfomer "transfoms" voltage and current (inversely to each other) but only "transfers" power.

On the primary side you may have 500 volts and 100ma which equals 50 watts. At that point you will have 50 watts on the output which will look like 20 volts and 2.5 amps at the 8 ohm tap.

The 500 volts have been transformed DOWN to 25, and the 100ma has been transfomed UP into 2.5A while the power has remained constant. You can see this is in proportion to the TURNS RATIO. As in the previous example, the turns ratio between the primary and the 8 ohm tap was "25:1". So, 500V/25=20Volts, and 100ma X 25 = 2.5A. 20Volts @ 2.5A into 8 ohms is 50 Watts.

Now the 50W possible at the primary side ONLY happens if the output tubes "see" the intended impedance. Since the 500V B+ is a fixed supply on the amp, and the output tubes are a fixed operation, then clearly what is variable is the "load" seen by the output tubes. Thus, the amp can put "50W" into the output side of the transformer WHEN the load on the output tubes is "5000 ohms".

When the load drops way below that, the CURRENT SWING will be limited, and when the load goes way above that the VOLTAGE SWING will be limited. Limiting either voltage or current restrains power.

Back to the speaker. So, if the speaker impedance swings from 4 ohms to 40 ohms so does the available power of the amp. At first, this would seem ridiculous as the speaker would then have a wildly fluctuating output "sound pressure." But, the drivers, which are exhibiting this "impedance swing" are also changing sensitivity that is roughly proportional.

So, as a crude example, the woofer when at "8 ohms" may require 10 Watts of power to produce 90dB SPL, but when it zooms to 40 ohms impedance (at resonance) it only needs 1W of power to produce the same 90dB SPL. So, even though the amplifier is "losing power" at the 40 ohm load, you don't NEED as much power for a constant SPL level. So the amp is "shock absorbering" the impedance swing.

SS amps (with no transfomer) are generally ALWAYS voltage limited by the rail voltage. So, they can easily handle loads as they go LESS than 8 ohms, because they can put out huge amounts of current. But when loads go ABOVE 8 ohms, the power will always drop because the rail voltage is very close to the output voltage.

Tube amps are the reverse because of the "transformer". They can deal with loads above the intended impedance since the B+ is hugely higher than the output voltage there is always room. But with loads BELOW the intended range they quickly run out of current. The maximum current in the ouput tubes is a tiny fraction of the load current.

This is generally why SS amps are referred to as current amps, and tube amps are voltage amps.
mdeneen

Careful, I'm beginning to understand here....

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On 5/18/2004 9:40:22 AM mdeneen wrote:

Tube amps are the reverse because of the "transformer". They can deal with loads above the intended impedance since the B+ is hugely higher than the output voltage there is always room. But with loads BELOW the intended range they quickly run out of current. The maximum current in the ouput tubes is a tiny fraction of the load current.


This is generally why SS amps are referred to as current amps, and tube amps are voltage amps.

mdeneen
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So to answer my question on the Forte/Heresy thread, if I hook the 2 nominal 4 ohm speakers together in series so that they present a "nominal" 8 ohm load on my 4 ohm taps, every thing should be ok? right?

To Mark Deneen,

Excellent explanations, just like my professors back in the day.

However, what sound characterisitics will I notice changing my LaScalas or cornwalls from the 8 ohm tap to the 4 ohm tap. I have Scott integrateds.

I am guessing that from what I have read here, the speakers flucuate more above 4 Ohms than below. Thus playing into the sweet spot of a tube amp. Or maybe better stated, playing into its capabilities.

So are you saying that you can get better sound on the 4 ohm tap with Klipsch speakers and tube amps? What are your experiences in percieved sound?

Thanks for any additional info here.

My Forte 1s are rated at 4 ohm on the nameplate and 8 ohm by Klipsch specs. I first played them at 8 ohm and they sounded good. I then played them on the 4 ohm tap and the bass became deeper and the mids became better also, while causing me to turn the gain up on the preamp a little higher to acheave the same sound pressure.

Will it hurt anything if I change just 1 speaker to the 4 ohm tap and leave the other on the 8 ohm tap temporarily? Then, I could hear the difference between the 2 Channels. The Scott integrateds have nice controls for balancing the channels and should work nicely to help determine the better tap.

At preacherman DaddyDee’s Klipsch gathering in Little Rock, Arkansas (May, 2004), Stream moved from the 8-ohm posts on his copper faced Cayin TA30 integrated tube amplifier to the 4-ohm taps. The difference on his unmodified RF7s was slight, but my brief impression was that the bass (because it was lower or because there was more of it?) extended further into the room, without any significant change in the mid-range…

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On 5/18/2004 10:46:26 AM mark1101 wrote:

Will it hurt anything if I change just 1 speaker to the 4 ohm tap and leave the other on the 8 ohm tap temporarily? Then, I could hear the difference between the 2 Channels. The Scott integrateds have nice controls for balancing the channels and should work nicely to help determine the better tap.
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I can't see how it would hurt anything, each channel has it's own amp so one shouldn't affect the other. With my amp, it is a solder reconnect so it was a bit more invloled.

"So to answer my question on the Forte/Heresy thread, if I hook the 2 nominal 4 ohm speakers together in series so that they present a "nominal" 8 ohm load on my 4 ohm taps, every thing should be ok? right?"

NO

Unless the speakers are identical the frequency response will be changed as the peaks and dips in impedance are at different frequencies.

"So are you saying that you can get better sound on the 4 ohm tap with Klipsch speakers and tube amps?"

At low volumes a speaker always plays loudest on the 8 and 16 ohm taps. As you turn up the volume and 'thump' it a bit you will find the speaker sounds the best on the tap that closest matches its impedance, and plays the loudest too.

Always try the 4 ohm tap. Leave it on whatever sounds the best.

Hint: it sounds 'thin' when you turn it up if you have it hooked to too high of a tap.